Mah, I don't know really...
I thought that "treated as a light weapon" was only for the purpose of 2WF. For example, if you use only one of the 2 ends of a double weapon (whichever you use), you don't treat it as a light weapon.
I mean: let's say you have a double sword, one end is flaming and the other is frost (I give them an enchantment so that is easy to distinguish between the two even if the are the same shape - both longsword - and to give a reason why a PC may use one end instead of the other).
Now, if you use only the flaming side (you don't get extra attacks for 2WF but neither the penalties) it is a 2handed weapon with all his benefits to Str bonus. If next round you only use the frost end, it is the same. With this I mean that the weapon is NOT a 1handed+1light weapon. You consider it like this only when you use both ends. If you don't use both ends in the same round, you definitely can't use weapon finesse. Unfortunately (because I think it's really lame...) the rules let you use the second end only for extra attacks, and if you don't take extra attacks you can't for example - if you have BAB +6/+1 - strike once with each, you must strike twice with the same end
When you do use both ends to get more attacks in the full-attack action, you treat them as a 1handed+1light weapon for two purposes: first for Str bonuses and second for 2WF penalties.
It is questionable at least if you must consider the 2nd end light for feats such as Power Attack and Weapon Finesse. I gave this example because the 2 ends of the weapon are identical, but unless you have all the 2WF feats (up to get the same number of attacks with each) every time you use both you choose which end is the main and which is the secondary (this may not be the same if the ends are different).
I am not sure, because by instinct I'd say that Finesse shouldn't apply (although the feat COULD have some benefit for 2WF, and since both are dexterity-based it would make sense). On the other hand, I'd also say that Power Attack shouldn't apply because it's very difficult to be fast (2WFing) and hard (PAing) at the same time, although you could still power-attacking with the 1st end (that is, take the penalty to ALL attacks and the bonus only to attacks with the 1st end).
But as a DM I would have to rule that either:
1) don't consider the 2nd end "light" for feats = Weapon Finesse NO, Power Attack (with both ends) YES
2) do consider the 2nd end "light" for feats = Weapon Finesse YES, Power Attack (with both ends) NO
If I had to choose I'd probably choose the 2nd, anyway.